1. Field of the Invention
In general, this invention relates to a sensor system for measuring the properties of a fluid. More specifically, this invention consists of a diffuse light source optically coupled to a transparent medium such that light rays beyond the critical angle at an interface between the transparent medium and a fluid are reflected back to a light sensor that is optically coupled to the transparent medium. Additionally, a light absorbing coating may be applied to the surface of the transparent medium to enhance the sensor's performance. The sensor allows the measurement of properties of the fluid such as refractive index and fluid mixture component concentration. An additional critical angle light “transmission” sensor may be incorporated for measuring light rays transmitted through the fluid that are less than the sensor system's critical ray angle. Light sensors for monitoring the “back scattering” of light, “forward scattering” of light, absorption of light, and transmission of light by the fluid medium may also be incorporated into the sensing system.
2. Description of Related Art
Real time measurement of fluid properties is important for the monitoring and control of processes. Refractive index is a property of a fluid that can be used to indicate the concentration or composition of a fluid that consists of multiple components. Some examples of processes in which fluid properties are of interest are refrigeration, food processing, juice dispensing, machining lubricants, battery and fuel cell electrolytes, and antifreeze mixtures.
Low cost, sensitive and reliable methods that can detect fluid mixture properties are desired. A variety of methods have been developed in order to determine a fluid's composition. Electrical conductivity can be used for fluids with electrolytic characteristics such as salt solutions. Polarimetry can be used for optically active fluids such as sugar-based fluids. Surface plasmon resonance can be used when a surface treated with a thin metallic coating, such as gold, can be directly exposed to the fluid. Coriolis force techniques and speed of sound techniques can be used when compositional changes result in fluid density changes. Viscosity methods can be used when the fluid's viscosity changes with compositional changes.
Refraction of light as it passes through a fluid is a method that can be utilized when variations of the fluid's composition or state affects the refractive index of the fluid. Some methods monitor the variation of a light beam's angle and/or displacement as it passes through the fluid. Other methods utilize the variation in the critical angle at an interface between the fluid and an adjacent material.
In addition to refractive index, determination of a fluid's light scattering and light absorption properties are also of interest. Methods that incorporate light scattering and light absorption measurement capabilities with refractive index measurement are desired. Dirt or other contaminants may increase the scattering and absorption characteristics of a fluid. Also, light scattering and absorption may indicate that an appropriate amount of additive, such as a pulp or fiber has been added to a system.